Should we fear an AGI if it will be equivalent to a human?
There are basically two worries:

If we create an AGI that is a slightly better AGI-programmer than its creators, it might be able to improve its own source code to become even more intelligent. Which would enable it to improve its source code even more etc. Such a selfimproving seed AI might very quickly become superintelligent. 

The other scenario is that intelligence is such a complicated algorithmic task, that when we finally crack it, there will be a significant hardware overhang. So the "intelligence algorithm" would be human level on 2030 hardware, but we figure it out in 2050. In that case we would immediately have superintelligent AI without ever creating human level AI. This scenario is especially likely because development often requires a lot of test runs to tweak parameters and try out different ideas.